It is known that water/ice exhibit a phase transition while passing through 0° C. In particular, the values of the dielectric properties exhibit a dramatic change as the temperature is lowered across the freezing point. Furthermore, the conductivity of liquid water changes dramatically with its ionic content. This allows for use for use of total impedance measurements to the presence of either water or ice on for example a surface of an aircraft skin.
U.S. Pat. No. 6,239,601 describes a device and method for determining the thickness of a layer of solid ice, a mixture of ice and water or a mixture of ice and other liquid. The device comprises two impedance sensors operated at different frequencies. Each impedance sensor comprises two conductors. An impedance measuring unit is associated to each sensor, said impedance measuring unit being arranged to produce an output voltage dependent upon the total impedance changes sensed by the sensor. A processor is coupled to the measuring units to generate an output value using the measured output voltages. The output value is indicative of the thickness of the ice or ice and water mixture or ice and other liquid. The impedance sensors are embedded in an embedding material formed in a cavity of a surface if an aircraft, along with a temperature sensor.
One disadvantage with the device described in U.S. Pat. No. 6,239,601 is that a cavity for the electrodes is formed in the aircraft skin. The cavity introduces surface roughness perturbing laminar flow of modern aircraft. Furthermore, the device is complex due to the use of a plurality of electrodes.
US 2009/0261811 describe a device for detection of ice using a surface mounted sensor that can detect changes in permittivity to sense the presence of ice. The change in dielectric properties of overburden water and/or ice is detected using homodyne detection or another frequency selective detection scheme. In homodyne detection, the signal to be detected is mixed with a reference signal of the same frequency. In one example a lock-in amplifier is used as a homodyne detector.
The device used in US 2009/0261811 is based on the needs to be wormed up to repeat the process of detection to acquire statistical confidence. Thus, the important time aspect is not handled by the device.